1. Field of Invention
The present invention relates generally to ablation catheter systems that use electromagnetic energy in the microwave frequency range to ablate internal bodily tissues. More particularly, the present invention relates to a monopole tip for a catheter that enables distal fire capabilities while enabling a relatively even electromagnetic field to be created at the sides of the monopole tip to facilitate the ablation of cardiac tissue.
2. Description of the Related Art
Catheter ablation is a therapy that is becoming more widely used for the treatment of medical problems such as cardiac arrhythmias, cardiac disrhythmias, and tachycardia. Most presently approved ablation catheter systems utilize radio frequency (RF) energy as the ablating energy source. However, RF energy has several limitations which include the rapid dissipation of energy in surface tissues. This rapid dissipation of energy often results in shallow "burns," as well as a failure to access deeper arrhythmic tissues. As such, catheters which utilize electromagnetic energy in the microwave frequency range as the ablation energy source are currently being developed. Microwave frequency energy has long been recognized as an effective energy source for heating biological tissues and has seen use in such hyperthermia applications as cancer treatment and the preheating of blood prior to infusions. Catheters which utilize microwave energy have been observed to be capable of generating substantially larger lesions than those generated by RF catheters, which greatly simplifies the actual ablation procedures. Some catheter systems which utilize microwave energy are described in the U.S. Pat. No. 4,641,649 to Walinsky; U.S. Pat. No. 5,246,438 to Langberg; U.S. Pat. No. 5,405,346 to Grundy, et al.; and U.S. Pat. No. 5,314,466 to Stern, et al., each of which is incorporated herein by reference in its entirety.
Cardiac arrhythmias, which may be treated using catheter ablation, are generally circuits, known as "reentry circuits," which form within the chambers of the heart. As is known to those skilled in the art, reentry circuits are abnormal electrical pathways that may form in various areas of the heart. For example, reentry circuits may form around veins and/or arteries which lead away from and to the heart. Cardiac arrhythmias may occur in any area of the heart where reentry circuits are formed.
The catheters used for treatment of cardiac arrythmias, disrhythmias, and tachycardia may have a variety of different antenna configurations to create electromagnetic fields used in ablation. Some catheters have antennas that essentially protrude from the distal ends of the catheters. In other words, some catheters have antennas which form the distal tips of the catheters. A monopole antenna is typically configured to form the distal tip of a catheter.
FIG. 1a is a diagrammatic representation of a distal end of a catheter with a monopole antenna at its tip. A distal end 102 of a catheter has a monopole antenna 108 at its tip. As shown, monopole antenna 108 has a rounded shape, and is coupled to a center conductor 112 of a co-axial transmission line 116. Typically, monopole antenna 108 is formed from a metallic material. Distal end 102 of the catheter may also include electrodes 120, which may be used for mapping processes, that may be coupled to processing equipment (not shown) using ECG wires 122.
Monopole antenna 108 is often arranged to be used in ablating tissue. Center conductor 112 transmits energy, e.g., electromagnetic energy, to monopole antenna 108 to allow an electromagnetic field to be formed with respect to monopole antenna. FIG. 1b is a diagrammatic representation of a monopole antenna, i.e., monopole antenna 108 of FIG. 1a, shown with electromagnetic field lines. Electromagnetic field lines 130 generally radiate from monopole antenna 108 in a substantially ellipsoidal pattern. Hence, near sides 134, "hot spots" 138 of electromagnetic energy are typically formed. Hot spots 138 are generally associated with the highest amounts of electromagnetic energy radiated by monopole antenna 108. The existence of hot spots 138 causes certain portions of a myocardium of heart, for example, such as those that are substantially contacted by a hot spot to be ablated more than other portions.
When an ablation procedure is performed using monopole antenna 108, the depth of cuts formed may not be uniform, since electromagnetic field lines 130 are not uniform. That is, the shape, or profile, of electromagnetic field lines 130 are such that when ablation is performed, the depth associated with the ablation may not be even. The lack of even depth in an ablation procedure may cause the ablation, e.g., an ablation in the myocardium of a heart, to be unsuccessful, as all of the cardiac tissue may not be effectively ablated. Hence, the ablation procedure may have to be repeated, which is both time-consuming and inefficient.
Therefore, what is needed is a monopole antenna structure for use with an ablation catheter that efficiently allows tissue to be ablated. More specifically, what is desired is a monopole antenna structure that is capable of producing a relatively uniform field, e.g., electromagnetic field, a deep lesion, and a microwave power deposition at the tip of a catheter, i.e., a tip-firing catheter.